1. Field of the Invention
This invention relates generally to Josephson junction circuits and is more specifically related to Josephson junction logic circuits. Still more specifically, it relates to logic circuits with spatially distributed inputs and outputs. The outputs, which are formed by portions of a wire-over-groundplane transmission line, are disposed in series with a plurality of Josephson junctions, each of which may have one or more input control lines. The circuit which is terminated at both ends with a resistor equal to the characteristic impedance of the transmission line is powered from a constant voltage power supply. Neither the fan-in nor the fan-out is limited by d.c. design considerations and, in common with other terminated line Josephson circuits, switching speeds in the tens of picoseconds are obtainable. As a result of this unique combination of features, the above described circuit is potentially applicable to picosecond array logic, read-only stores, decoder-driver arrays, memory sense lines, data busses and carry look-ahead networks, as well as to random logic networks.
2. Description of the Prior Art
Josephson junction devices and Josephson junction logic circuits are well known in the prior art. The basic theoretical explanation of the Josephson effect is given in an article "Possible New Effect in Superconductive Tunneling" by B. Josephson, published in Physics Letters, July 1962, pp. 251-253. Since then, numerous other prior art references disclosing Josephson devices and showing their applications have been published. For example, J. Matisoo in an article entitled "The Tunneling Cryotron -- A Superconductive Logic Element Based on Electron Tunneling" in Proceedings of the IEEE, Vol. 55, pp. 172-180, February 1967, discusses early work which uses Josephson tunneling devices in the logic environment. More recently, D. J. Herrell, in an article entitled "Femtojoule Josephson Logic Gates", published in the IEEE Journal of Solid State Circuits, Vol. SC-9, pp. 277-282, October 1974, shows a demonstrated applicability to very fast logic operating at cryogenic temperatures.
A terminated line logic circuit (TLL) is shown in U.S. Pat. No. 3,758,795 in the name of Anacker et al, issued Sept. 11, 1973, and assigned to the same assignee as the present invention. In this patent, a single Josephson junction fed from a current source is shunted by a transmission line which is terminated at its center by a series resistor of resistance twice the characteristic impedance of the transmission line. This circuit has a fan-in of only two or three and practically unlimited fan-out capabilities. It is properly terminated only for differential mode signals originating at the gate. As a consequence, only signals which are exactly equal and opposite in polarity at the terminating resistor are properly terminated. Thus, the delay must be equal on the two lines which extend between the logic gate and the terminating resistor. As a consequence, only one gate is used in each logic stage. Logic is generally performed by three controls placed side by side over the gate and fan-in is limited to two (for inverters) or three control lines localized to the gate. The two lines needed to carry signals from the gate to the most remote output impose a burden on wireability which is substantially eased by the present circuit.
U.S. Pat. No. 3,458,735, entitled "Superconductive Totalizer or Analog to Digital Converter" which issued on July 29, 1969 to M. D. Fiske shows a plurality of Josephson junction devices arranged in series which are fed from a constant current source. Since Fiske is detecting the sum of the voltage drops across the series string of Josephson junctions, it is clear that he does not utilize a constant voltage source similar to that called for by the present application.
An IBM Technical Disclosure Bulletin entitled "Josephson Junction Circuit" by G. J. Lasher, Vol. 11, No. 10, March 1969, p. 1222, shows a Josephson junction in the transmission line environment which is terminated with the characteristic impedance of the strip line so that no reflected signal returns to the junction due to its own a.c. emission. This publication is concerned with improving the bistable behavior of a Josephson junction and is not concerned with a Josephson device having a control line; the voltage states of which are communicated to output portions which, in turn, are coupled to other Josephson junctions.
Similarly, IBM Technical Disclosure Bulletin, Vol. 15, No. 3, August 1972, p. 899, in an article entitled "Josephson Junction Circuits Having Magnetic Feedback" by H. H. Zappe shows in FIG. 2 a single Josephson device controlled by a plurality of associated control lines. This publication, like the Lasher publication, shows only one device and, while there is a suggestion in Fiske that devices may be placed in series, there is no suggestion or indication that such serially arranged devices can be substituted for the single device of Lasher to achieve a logic circuit wherein a plurality of Josephson junctions can be disposed in a substantially random fashion and appear as a properly terminated logic circuit for wave fronts originating from the switching of any of the serially disposed Josephson junctions. The distributed Josephson junction logic circuit of the present application is believed to be distinguishable and unobvious over all the above cited prior art in that it unexpectedly provides a logic circuit which has superior fan-in capabilities relative to known circuits and practically unlimited fan-out capabilities when a transmission line containing serially disposed Josephson junctions and serially disposed control line portions is terminated at both ends in the characteristic impedance of the transmission line and energized at one end thereof by a constant voltage source. The particular arrangement of devices, matching terminating impedances and a constant voltage source provides a flexibility heretofore not obtainable using prior art arrangements.